Global Workpiece Positioning System (GWPS) part 1: Concept and development of a laser probe based system

One component of a modern machining system that has remained virtually unchanged over time is part location. Current methods of part location use physical datum surfaces on the workpiece. Workpieces are positioned relative to the axial reference frame of the machine tool by placing these datum surfaces in contact with locators in the fixture. Due to datum surface variation and fixturing errors, workpiece positioning is often inconsistent and variable for machining operations requiring multiple setups. In turn, this leads to datum-related geometric errors of the machined surfaces. This paper presents a potential solution to these problems in the form of the Global Workpiece Positioning System (GWPS) concept. Using the GWPS concept, artifacts attached to the workpiece are used to define a system of datum target points that, in turn, are known relative to a global workpiece reference frame fixed to the workpiece. Using a target point sensing process, the locations of subsets of artifact target points are mapped into the axial space of the machine tool with a level of uncertainty equivalent to the axial motion uncertainty of the machine tool. In this way, the location of the global workpiece reference frame with respect to the machine tool axial reference frame is defined. All machining operations for all setups are performed relative to this reference frame. This paper, which is part one of a two-part paper, describes the GWPS concept. It further describes the development, testing, and evaluation of a laser probed based GWPS. The average positioning accuracy of this system was determined to be 1.3 mu m, which is within the axial motion uncertainty of the machine tool on which it was implemented. This measure is based on the location of a drilled and reamed hole. The second paper describes the implementation and testing of the GWPS with ultrasonic transducers and receivers. This system utilizes time-of-flight ranging and is similar in principle to the current satellite-based Global Positioning System (GPS).